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Medium-term Wall Assembly for Shed conversion to occasional camp/cabin use

JRR | Posted in General Questions on

Bought a property with existing shed with 2×4 walls with (from outside to inside) vinyl siding/tar paper/osb then open to the inside.  Zone 4/5 (Northern Shenandoah Valley) with electricity only (no plumbing planned).  12×16, 1.5 level, gambrel roof.  Eventually, when the vinyl siding is closer to the end of its useful life & I have more of a budget for a larger renovation, I’m thinking to pull the siding and the tarpaper in order to housewrap then rigid foam (4″?), then re-side with vinyl and, as far as the interior of the sheathing, probably address the 2×4 voids with mineral wool and maybe a smart vapor retarder like Membrain, then “faux tongue and groove” plywood panels for inside wall covering.  For now though, is there anything I can do to make the shed/cabin do better for climate control when we’re there on the weekends? (a couple weekends per month).  What about double bubble foil wrapping horizontally across all the interior studs, taping those seams, then running a small dehumidifier constantly?  Seems to me attempting mineral wool in stud wall, at this point, could cause problems with moisture since the r-value would be insufficient to keep moisture out/keep it dry until I am ready to do the foamboards on the exterior of the sheathing.  Thoughts?

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Replies

  1. Expert Member
    Michael Maines | | #1

    The same rules apply as for any home: the best bang-for-the-buck is addressing air leaks, while keeping bulk water out and controlling (or at least understanding) vapor movement. Bubble wrap can be taped to function as air control and it blocks vapor movement, but its R-value is miniscule. Depending on interior and exterior conditions, bubble wrap alone could lead to moisture accumulation.

    I would seal any potential air leaks with a flexible sealant (Pro Clima Contega HF is a good one) and/or tape. Save up for doing the insulation once, and doing it right. If you do it half-assed now, you are much less likely to go back and do it properly later.

    1. JRR | | #3

      Thank you Michael, Do you agree that just doing the stud bays with mineral wool at this point would be a poor idea considering my plans to do exterior rigid XPS/EPS later? I was thinking that just the mineral wool would need a vapor retarder between it and the sheathing but then, once I do the exterior foam board, the sheathing will be sandwiched with moisture sealed in. I only want to move so far ahead such that I won't need to pull anything out and redo it later on, when I finally complete the process. Thoughts?

      1. Expert Member
        Michael Maines | | #10

        It's hard to predict what will happen with spaces that aren't fully conditioned or fully unconditioned. Vapor drive can be in either direction in your climate zone, and varies with temperature and humidity. Insulating the walls with mineral wool might work fine, or it might not. If you will be heating the space in cold weather, you definitely don't want vapor control on the outside of cavity insulation, unless it's in the form of foam thick enough to provide dewpoint control.

        1. JRR | | #11

          Thank you, Michael. I do plan heating in cold weather. I'm interpreting your synthesis to be that, if avoiding mistakes is a primary criterion, it sounds like there is no part of the insulation/vapor control that I should attempt. I'll have to simply install the rodent hardware fabric and control the air movement/radiant heat when occupied and then keep it vapor open when unoccupied. Does that sound right?

  2. Deleted | | #2

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  3. DC_Contrarian | | #4

    Unoccupied buildings have a special problem. If they are built the way high-performance buildings are normally built -- insulated, air-sealed and with a vapor barrier -- warm humid air can get trapped in them when they are occupied or through infiltration in long periods of warm weather. When it gets cold outside the inside cools off and the moisture condenses.

    The solution is to ventilate when the building is unoccupied.

    1. JRR | | #5

      Thank you, DCC, for your angle. Are you mentioning this because I indicated foiled bubble wrap for the near term? I was thinking to make sure to leave the windows and doors area uncovered during the unoccupied periods which, from my understanding, would return air exchange with the outside to provide ventilation. In my original post, I also mentioned the willingness to utilize a small dehumidifier (w/ a through-wall drain hose for continuous function) if it would be advisable. Do you think one of these options would adequately address the infiltration factor? Thank you

      1. DC_Contrarian | | #6

        I don't know how to predict. If you want to freak yourself out, google "shipping container condensation."

        By "uncovered" for the windows and doors do you mean open? You want them screened to keep out pests. But then you have to worry about a racoon or something tearing the screen.

        1. JRR | | #8

          Hi DCC, by "uncovered," I was referring to the frequent reference of how windows and doors are commonly leaky. I had the impression that, as long as I didn't include the window/doors in my bubble foil encapsulation, they would provide enough air exchange for a structure this small. I could design in some sort of flap that could be left open when I wasn't there, to let the bubble wrap enclosure air spill out, if you think that would be necessary. Thoughts?

  4. tjanson | | #7

    Is there a ceiling or it is open rafters? (i.e. what's the plan for insulating that?) How big is this place? What is your heat source? What is the floor construction and foundation?

    I think the bubble wrap is a waste of time. I doubt you'd see much of an improvement in
    comfort or heat retention. Adding batts to the wall without anything covering the interior side can cause condensation of interior moisture on the sheathing when you are heating and occupying the space. So batts + interior vapor retarder could be a decent first move.

    1. JRR | | #9

      Q1. Is there a ceiling or it is open rafters?(i.e. what's the plan for insulating that?)
      -A: Open rafters. Plan is: medium term bubble foil, long term is Lstiburik vented roof with rigid foamboard (offset seams).
      Q2. How big is this place? A: 12x16, 1.5 story gambrel roof.
      Q3. What is your heat source? A: TBD once encapsulated.
      Q4. What is floor construction /foundation? A: Plywood subfloor, 2x8 joists,2-3' piers.
      Tjanson Stated: "Adding batts to the wall without anything covering the interior side can cause condensation of interior moisture on the sheathing when you are heating and occupying the space. So batts + interior vapor retarder could be a decent first move." Follow up Q/response: Enough moisture from a couple weekends a month that would have trouble dissipating the rest of the month? Can you address my stated option of the small dehumidifier? I'm concerned that putting vapor retarder in a wall assembly now, knowing I will eventually be putting foamboard outside the sheathing will break the standard I set of not having to disassemble anything once completing the long-term plan. Thoughts?

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